Write down a basic grading percentile scale of: 90-100 is an A; 80-89 is a B; 70-79 is a C; 60-69 is a D; and 59 or below is an F.
Calculate 10 percent of the total points on the test. If there are 80 potential points possible, figure out 10 percent of 80, which is eight.
Place a point scale next to your percentile/grade scale. In this case, where 10 percent of 80 is eight, then 72 to 80 points on the test is an A; 64 to 71 points is a B, and so forth, subtracting eight points down the scale.
Decide if you want to curve the test according to the highest score a student received rather than total possible points. If your best student received a 71 on the test, and you think it unfair to give only him an A, then you can curve the grading scale.
Curve it by making the highest student score the new base number. 71 now becomes the 100 percent (not 80). So, you subtract 7.1 points for the As. If you round that to a seven, any student receiving 64 to 71 points gets an A, and so on, subtracting seven points down the scale.